This invention relates to a method and an apparatus for detecting broken rails.
On many railway lines the presence of a train on a section of track is detected by means of a track circuit, which applies a low voltage between the rails, and detects the change in the resistance between the rails due to the presence of the train as the wheels and axles provide electrical connection between the rails. Track circuits incidentally also enable any break in a rail to be detected. There are however many railway lines in which track circuits are not used, and, especially on such railway lines, a method of detecting any break in a rail would be desirable and conducive to safer operations.
According to the present invention there is provided a method for detecting a break in a rail in a situation where two rails extend parallel to each other along a railway line, the method comprising connecting the two rails together electrically with a first electrical connection at a first location, and also connecting the two rails together electrically with a second electrical connection at a second location spaced apart from the first location along the line, the first electrical connection being connected to a source of electrical current, and the second electrical connection being connected to the current source via a return current path that does not form a part of the same track as either of the rails, so as to cause electrical currents to flow in parallel along the two rails between the first location and the second location, and either (a) detecting any difference between the currents flowing in each of the rails, and hence determining if there is a break in one of the rails, or (b) detecting the currents flowing in each of the rails, and from the two values of current determining if there is a break in one of the rails.
A break in either of the rails in the section of the line between the first location and the second location can hence be detected. Preferably the currents flowing in each of the two rails are detected, and the two values of current are used in determining if a break is present. The currents may be measured in the rails themselves, or more preferably may be measured in electrical connections leading to the rails, for example in the first or the second electrical connection. The currents may be direct, alternating, or pulsed. Preferably the currents have a frequency spectrum in which most or all of the energy is at low frequencies, preferably no more than 20 Hz (because the impedance of the rails increases with frequency). Such low frequency currents may be measured using a non-contact current sensor such as that described in WO 00/63057, but alternative current sensors may also be used.
There is thus an electrical circuit comprising the current source and the two parallel rails, with one side of the current source connected to the first electrical connection and the circuit being completed by the return current path. The return current path may be provided either by an electrical conductor connected between the other side of the current source and the second electrical connection, or by connecting both the current source and the second electrical connection to earth. The method is applicable to tracks that have no track circuits; and (unlike a track circuit) the sensor currents in the rails flow in parallel, so that if there is no rail break there is no voltage between the rails. In the preferred arrangement the two rails form a track for a railway vehicle, but in a multitrack line the two rails may instead be in different tracks.
Preferably the interpretation of the two values of current involves a comparison of at least one of the values with a first threshold value, to indicate if the current is sufficiently large for reliable operation; and also a comparison between the two measured values, to see if the difference between the measured currents exceeds a second threshold value indicating that there is a break in one of the rails. This second threshold value may be a preset proportion of one of the measured values of current, or of the sum of those measured values, and so be related to the current supplied by the current source. As indicated above, the currents may be measured within electrical connections leading to the rails; they may also be measured in such electrical connections at both ends of the section of line.
The invention also provides a system for detecting a break in a rail operating as described above.
Successive sections of the rails, along the line, may be electrically insulated from each other, and each section provided with a separate detection system; each detection system can then operate independently of the others. If that is not the case, so that successive sections of the rails are in electrical contact with each other, then each section may be provided with a separate detection system, and the separate detection systems activated in turn (so that nearby detection systems are not activated at the same time); this again allows each detection system to operate independently. Alternatively each detection system may operate with an alternating current, or a pulsed current, so the currents from nearby detection systems can be distinguished from each other for example by their frequencies. In a preferred embodiment each detection system operates with a pseudo-random pulsed current, the pseudo-random currents having a different pattern in adjacent detection systems; in this case cross-correlation between the observed currents and the expected pseudo-random pulse sequence enables the currents from adjacent detection systems to be distinguished.